The invention relates to a bushing for leading at least one cable through an opening in a wall or the like whilst providing protection against electromagnetic overtension and/or interference.
Such a bushing is known from Dutch patent application NL 173.226. This known bushing comprises an electrically conductive wall bushing and a protective circuit against overtension incorporated therein, which comprises at least two conduction components for each supply and return cable core to be connected from outside, and which terminates in terminal connecting points for said cable cores. A cross partition connected to said wall bushing divides said components into two departments, wherein all components of the protective circuit and the terminal contact points of the supply and return cable cores are placed on an electrically insulating circuit board. Said circuit board is fitted in the cavity of said wall bushing as a prefabricated insert unit, and fixed thereto together with said cross partition.
One drawback of the known bushing is that it is not chemically resistant, fire resistant, gastight and watertight. Another drawback of the known bushing is the fact that its construction is complicated and expensive. Furthermore, this known bushing hardly offers a possibility to inspect the protective circuit in the mounted condition of the bushing.
The objective of the invention is to provide a bushing comprising a protection against electromagnetic overtension and/or interference, which is simple, inexpensive, chemically resistant, fire resistant, gastight and watertight, and whose protective device can easily be inspected in mounted condition.
A bushing of the kind referred to in the introduction is according to the invention characterized in that said bushing comprises a sealing collar of an elastic material, which can be sealingly fitted in the opening in the wall, and which is provided with at least one through channel for sealingly leading said cable therethrough, which sealing collar is electrically conductive and which is in contact with an electrically conductive cable shield of the cable in the operative condition of the bushing. The term electrically conductive sealing collar which is used in this framework is understood to mean that the material of the sealing collar is at least substantially electrically conductive, in particular in that additives have been added to the sealing material (preferably a resilient material such as rubber). Electrical conductivity has been realized by chemical means, therefore. Electrical installations and machines, wherein generally highly sensitive components are used for control and registration purposes, are highly susceptible to interference caused by various external electromagnetic fields and impulses. Said external electromagnetic fields and impulses may for example be generated by radar installations, thunderstorms, nuclear explosions and by electric/electronic equipment. In practice, steel constructions functioning as a Faraday cage are often placed round a space intended for electronic equipment in order to minimize such interference. Nevertheless, there is still a chance that electric cables, which are led into such a shielded space, will conduct undesirable electromagnetic fields and impulses into said electronic equipment. As a rule, said conduction will take place via the electrically conductive shielding (xe2x80x9cbraidingxe2x80x9d) of the bushed cables in question. According to the invention, said external, undesirable electromagnetic fields and impulses will flow to earth along the electrically conductive shielding of a bushed cable, via the electrically conductive sealing collar. Thus, the occurrence of said interference in electronic equipment is prevented in a simple manner. Furthermore it has become apparent that the present bushing is also quite suitable for leading a cable through a steel deck or bulkhead of a ship.
It is noted that within the framework of the invention, the term xe2x80x9ccablexe2x80x9d as used herein is understood to include an electrically conductive conduit, tube, pipe or the like, for example a metallic tube or conduit, with which the present sealing collar is in electrically conductive contact in the operative condition.
In one embodiment of a bushing according to the invention, the sealing collar abuts at least partially against the electrically conductive cable shield of the cable in the operative condition of the bushing. This makes it possible to fit the sealing collar easily and firmly round the electrically conductive cable shield.
In another embodiment of a bushing according to the invention, said sealing collar is made of (massive) rubber, to which carbon particles in the form of soot (xe2x80x9ccarbon blackxe2x80x9d) have been added. Preferably, said rubber is EPDM rubber, with the carbon particles in particular having a concave, dish-like shape, preferably being of the type xe2x80x9cKetjenblackxe2x80x9d EC 300J or 600JD(trademark). These types of soot, which are known under the trade name xe2x80x9cKetjenblackxe2x80x9d, have a unique morphology in comparison with conventional types of soot, which makes them uniquely suitable for the present electrically conductive application. The letters EPDM stand for Ethylene/Propylene/Diene/Monomer.
In another embodiment of a bushing according to the invention, said sealing collar is provided with circular internal and external ribs, which external ribs have a diameter which is larger than the inside diameter of the opening in the wall, and wherein the inside diameter of the internal ribs is at least at the most substantially the same as the outside diameter of the cable. Thus, a reliable seal is realized between the sealing collar and the wall opening on the one hand and between the sealing collar and the cable on the other hand.
In another embodiment of a bushing according to the invention, the rear flanks, seen in the direction of insertion, of the outer serrated ribs extend in radial direction, and the inner ribs are provided in pairs in the radial area of the outwardly extending tops of the outer serrated ribs. Preferably, the inner ribs, seen in the direction of an axial longitudinal section, are trapezoidal. Thus, an optimum sealing effect is realized, wherein the serrated ribs are slightly compressed against the wall opening.
In another embodiment of a bushing according to the invention, the sealing collar is made in one piece.
In another embodiment of a bushing according to the invention, the sealing collar comprises two shell parts, whose abutting boundary surfaces extend at least substantially in axial direction. In the operative condition, a pressure is thereby generated on the axial boundary surfaces, such that a reliable seal is formed between said two boundary surfaces.
The invention furthermore relates to a method for manufacturing a bushing according to the invention, wherein said sealing collar is made of an electrically conductive rubber by incorporating concave, dish-like carbon particles into the rubber by means of a vulcanizing process. The rubber is in particular EPDM rubber, whilst the carbon particles are preferably of the type xe2x80x9cKetjenblackxe2x80x9d EC 300J or 600JD(trademark).